Fracture-filling cement in the Boquillas Formation: A proxy for the origin and maturation of hydrocarbons in South Texas

Abstract

The Austin Chalk is a low porosity (3–10%) and low permeability (0.1–0.5 mD) reservoir that relies on fracture networks to store and produce hydrocarbons at commercial flow rates. Charged by the underlying Eagle Ford Formation, fracture networks provide critical permeability and fluid migration pathways from the Eagle Ford source to the Austin Chalk reservoir. In order to investigate these migration pathways, this study analyzes outcrops of the Eagle Ford Formation, located along U.S. 90, west of Del Rio (Texas), also known as the Boquillas Formation. These outcrops expose two sets of conjugate hybrid-shear fractures whose dilational segments are filled with calcite veins. Calcite veins microstructure suggest that they experienced repeated crack and seal events and contain abundant fluid inclusions. Fluids include aqueous solutions and liquid hydrocarbons. Micro-thermometry analyses of aqueous fluid inclusions suggest that they were entrapped at a depth of 1.8–2.7 km. Kinematic analysis suggest that the hybrid shear fractures predate oil generation and expulsion of the Eagle Ford Formation, allowing for migration of oil and gas in the fracture system immediately after generation. These results highlight the importance of these hybrid shear fractures as viable migration pathways for hydrocarbon migration in the Austin Chalk-Eagle Ford hybrid petroleum system.